Heat is often created as a byproduct of industrial processes where flowing streams of liquids, solids, or gasses that contain heat must be exhausted into the environment or otherwise removed from the process in an effort to maintain the operating temperatures of the industrial process equipment. Sometimes the industrial process can use heat exchanging devices to capture the heat and recycle it back into the process via other process streams. Other times it is not feasible to capture and recycle this heat because it is either too low in temperature or there is no readily available means to use as heat directly. This type of heat is generally referred to as “waste” heat, and is typically discharged directly into the environment through, for example, a stack, or indirectly through a cooling medium, such as water. In other settings, such heat is readily available from renewable sources of thermal energy, such as heat from the sun (which may be concentrated or otherwise manipulated) or geothermal sources. These and other thermal energy sources are intended to fall within the definition of “waste heat,” as that term is used herein.
Waste heat can be utilized by turbine generator systems which employ thermodynamic methods, such as the Rankine cycle, to convert heat into work. Typically, this method is steam-based, wherein the waste heat is used to raise steam in a boiler to drive a turbine. However, at least one of the key short-comings of a steam-based Rankine cycle is its high temperature requirement, which is not always practical since it generally requires a relatively high temperature (600° F. or higher, for example) waste heat stream or a very large overall heat content. Also, the complexity of boiling water at multiple pressures/temperatures to capture heat at multiple temperature levels as the heat source stream is cooled is costly in both equipment cost and operating labor. Furthermore, the steam-based Rankine cycle is not a realistic option for streams of small flow rate and/or low temperature.
The organic Rankine cycle (ORC) addresses the short-comings of the steam-based Rankine cycles by replacing water with a lower boiling-point fluid, such as a light hydrocarbon like propane or butane, or a HCFC (e.g., R245fa) fluid. However, the boiling heat transfer restrictions remain, and new issues such as thermal instability, toxicity or flammability of the fluid are added.
To address these short-comings, supercritical CO2 power cycles have been used. The supercritical state of the CO2 provides improved thermal coupling with multiple heat sources. For example, by using a supercritical fluid, the temperature glide of a process heat exchanger can be more readily matched. However, single cycle supercritical CO2 power cycles operate over a limited pressure ratio, thereby limiting the amount of temperature reduction, i.e., energy extraction, through the power conversion device (typically a turbine or positive displacement expander). The pressure ratio is limited primarily due to the high vapor pressure of the fluid at typically available condensation temperatures (e.g., ambient). As a result, the maximum output power that can be achieved from a single expansion stage is limited, and the expanded fluid retains a significant amount of potentially usable energy. While a portion of this residual energy can be recovered within the cycle by using a heat exchanger as a recuperator, and thus pre-heating the fluid between the pump and waste heat exchanger, this approach limits the amount of heat that can be extracted from the waste heat source in a single cycle.
Accordingly, there exists a need in the art for a system that can efficiently and effectively produce power from not only waste heat, but also from a wide range of thermal sources.